QOS doesn't give any stream preferential treatment. QOS guarantees certain level of bandwidth at all times for certain streams. Typically, it is reserved for things like VOIP where buffering causes issues. I don't think Netflix is deserving of QOS, but rather it deserves better guaranteed base because it can adjust with larger buffers, taking care of intermittent studdering due to temporary spikes in bandwidth usage.
The problem here, is that total aggregate bandwidth is simply being manipulated at peering points due to greedy ISPs like Comcast and AT&T. The congestion at peering points is simply a means to an end, and Comcast (et al) are taking advantage of stupid consumers. I can assure you that both Netflix (and similar) and the ISPs know exactly where the problem lies, AND how to solve it. And seeing what Netfix has offered the ISPs in terms of peering, and the lack of acceptance by the ISPs, the problem lies with ISPs and only ISPs.
Netflix has capacity, the ISPs have Capacity, but they can't agree on peering, which is simply the bridging of the two capacities. And knowing that they both have Routers in the same COLO facility with capacity, but the fiber connection between the two routers is missing, due to ISPs unwillingness to play fairly, is criminal.
From what I've heard, Netflix is willing to buy Comcast the router, fiber and all the bits needed to add additional peering bandwidth, and Comcast has refused, instead is trying to bully Netflix. Netflix needs to put up advertising "don't blame us" with a simplified version of what is happening. 30 Seconds is all it will take.